The present invention generally relates to electrical lead assemblies in devices such as electric lamps for providing an electrical path through a hermetic press or pinch seal formed in a vitreous material such as fused silica or hard glass. More specifically, the present invention relates to such assemblies having a metallic foil with an oxidation-protective coating on at least a portion of the foil.
In certain devices, it is often necessary to provide an electrically-conducting path through a pinch or press seal formed in a vitreous material. For example, in devices such as electric lamps, e.g., halogen incandescent filament bulbs and high intensity discharge arc tubes, a light emitting chamber is formed from a vitreous material having one or more pinch seals that hermetically seal the chamber. In such lamps, one or more electrically-conducting paths from the interior of the chamber to the exterior of the chamber are typically formed by positioning an electrical assembly in one or more of the portions of the tube, and “pinching” the tube to form a hermetic seal around a portion of the assembly. The electrical lead assembly typically includes a metallic foil having electrically conducting leads mechanically secured to the foil and extending from each end thereof. The assembly is positioned so that the foil forms the electrically conducting patch through a portion of the vitreous material that has been pressed together to form a hermetic seal.
Although any suitable material may be used, typically, the foil in such electrical lead assemblies is formed from molybdenum because of its stability at high temperatures, relatively low thermal expansion coefficient, good ductility, and sufficient electrical conductivity. However, molybdenum oxidizes rapidly when exposed to oxygen at temperatures greater than about 350° C. Since the foils in electrical lead assemblies in electric lamps are often exposed to temperatures greater than about 350° C., the metallic foil may be highly susceptible to oxidation resulting in a breach of the electrical path or the gas-tight integrity of the hermetic seal resulting in lamp failure. Typically, a molybdenum foil exposed to a reactive atmosphere will not oxidize appreciably below about 350° C. At temperatures greater than about 350° C., the rate of the reaction between the oxygen in the surrounding atmosphere and the molybdenum foil greatly increases resulting in corrosion of the foil and a substantial reduction in the useful life of the lamp. Areas particularly susceptible to such oxidation include the spot weld connecting the outer lead to the foil and the area on the foil adjacent the outer lead.
FIG. 1a is a schematic representation of a conventional arc tube for a high intensity discharge lamp. Referring to FIG. 1a, the arc tube 100 is formed from light transmissive material such as quartz. The arc tube 100 defines a chamber 110 formed by pinch sealing the end portions 115,120. An electrode assembly 122,124 is sealed within each end portion 115,120 to provide an electrically-conducting path from the interior of the chamber 110 to the exterior of the chamber through each end portion 115,120. Each electrode assembly 122,124 for a high intensity discharge arc tube 100 typically includes a discharge electrode 125,130, electrode leads 140,135, metallic foils 145,150, and outer leads 155,160. The electrode leads 135,140 and the outer leads 155,160 are typically connected to the metallic foils 145,150 by spot welds.
FIG. 1b is an illustration of the cross-section of a typical metallic foil 145,150 in an electrical lead assembly 122,124. As shown in FIG. 1b, the typical foil 145,150 is shaped in cross-section so that the thickness of the foil is greatest at the lateral center thereof, and reduces outwardly to each of the longitudinal edges. This shape has been found to reduce residual strain in the vitreous material that has been compressed around the foil during the high temperature pinching process and subsequently cooled. In a typical electrical lead assembly for an electric lamp, the foil may have a width of about 2 to 5.5 mm with a centerline thickness of about 20 to 50 μm and an edge thickness of about 3 to 7 μm. For example, a foil having a width of about 2.5 mm would typically have a centerline thickness of about 24–25 μm and an edge thickness of about 3 μm.
The assemblies 122,124 are positioned in the end portions 115,120 so that the foils 145,150 are pinched between the compressed portions of the end portions 115,120 forming the hermetic pinch seals. The assemblies 122,124 provide the electrically conducting paths through the each end portion 145,150 with the relatively thin foils 145,150 providing a current path through the hermetically sealed pinch regions.
The electrode lead assemblies provide a point of failure in such lamps due to corrosion, e.g., oxidation, of the metallic foils when exposed to corrosive agents such as oxygen at high temperatures. The assemblies 122,124 are particularly susceptible to oxidation at the outer portion of the foil 145,150 adjacent the outer lead 155,160 due to the exposure of this portion of the foil to oxygen or other corrosive agents during operation of the lamp. The oxidation may progress inward placing a significant amount of stress on the pinch seal. The stress may be evident from Newton rings or passageways which appear at the point at which the leads are welded to the molybdenum foil. Eventually, the electrical path may be breached or the pinch seal may crack causing the lamp to fail.
One reason for this failure is that during the formation of a pinch seal or vacuum seal with a vitreous material such as quartz, the quartz does not completely seal to the relatively thicker outer and inner lead wires, due at least in part to the relatively high viscosity of the quartz. Microscopic passageways may also be formed along the outer leads 155,160 and also along the outer edge of the foliated portion perpendicular to the transverse axis of the lamp due to the substantial difference in the coefficient of thermal expansion of the quartz compared to that of the refractory metal outer lead wire, which is typically tungsten or molybdenum. Efforts have been made in the past to prevent the oxidation of molybdenum foils in electrical assemblies that may be exposed to oxygen at high temperatures.
Various techniques have been suggested for inhibiting the oxidation of metallic foils, and particularly molybdenum foils. For example, it has been proposed to reduce oxidation by coating the molybdenum foil with oxidation-protective materials such as phosphides, aluminides, lead oxide, silicon nitride, alkali metal silicate and chromium. However, these prior art coatings are not desirable because the coatings are relatively thick and do not bond well to glass. Therefore, the prior art coatings must be applied to the exposed portions of the foil after the pinch or shrink sealing process is completed. The utility of the prior art coatings is also limited because the coatings cannot be exposed to high operating temperatures. A need remains for oxidation-protected metallic foils for use in electrical lead assemblies for providing electrically-conducting paths through pinch seals in vitreous material and that can be exposed to high operating temperatures.
Therefore, it is an object of the present invention to provide electrical lead assemblies that obviate the deficiencies of the prior art.
It is another object of the present invention to provide metallic foil that is protected from corrosion when exposed to corrosive agents at high temperature.
It is another object of the present invention to provide high intensity discharge lamps and/or halogen lamps with increased useful life.
It is still another object of the present invention to provide a process for coating a metallic foil to inhibit oxidation of the foil in reactive atmospheres at high temperatures.
It is yet another object of the present invention to provide a metallic foil for use in high intensity discharge lamps and halogen lamps which is oxidation protected.
It is a further object of the present invention to increase the life of devices by coating the metallic foil of electrical lead assemblies with various compositions to protect the foil from corrosion.
It is still a further object of the present invention to provide an electrical lead assembly having an outer lead formed by extending the metallic foil.
It is yet a further object of the present invention to provide an electrical lead assembly having mechanical attachment of an outer lead to a metallic foil with no welds.
It is yet a further object of the present invention to increase the life of the high intensity discharge lamp significantly, while reducing the manufacturing cost and the number of assembly parts.
It will be noted that although the present invention is illustrated with these and other objectives, that the principles of the invention are not limited thereto and will include all applications of the principles set forth herein.
These and other objects can be realized by simultaneous reference with the following non-exhaustive illustrative embodiments in which like segments are numbered similarly.